System and method for stacking receiver channels for increased system through-put in an RF data transmission system

ABSTRACT

A wireless RF data transmission system has at least one base station transmitting control information on a control channel and high speed data on at least one data channel. At least one subscriber station receives the control information and data channels, adjusting a modulation and/or coding of reception of the data in response to the control information. Preferably, the control information is transmitted using a more robust modulation, such as QPSK, than used to transmit the data, which is preferably transmitted using either 16, 64 or 256 QAM. Additionally, timing or header information for the data may be included on the control channel.

TECHNICAL FIELD

The present invention relates to RF data transmission systems andspecifically to a system and method for stacking receiver channels forincreased system through-put in an RF data transmission system.

BACKGROUND

Currently, there are several so-called “last mile” and “last foot” datatransmission systems which are designed to deliver high speed and/orhigh data capacity from the internet backbone to an end user. Severalsuch systems use RF transmissions to replace copper wire or fiber opticcables. Some of these systems are called point-to-point orpoint-to-multipoint systems and operate in various licensed andunlicensed RF bands. A fundamental characteristic of many such existingsystems is that their RF transmissions occur in a frequency spectrumprotected and regulated by a government body. These protected frequencyspectrums, or bands, are licensed to certain license holders and only afew may operate in any given physical area depending upon the number oflicenses available.

There are only a limited number of licensed bands in any geographicarea, thus, in order to widen the choices consumers have, it isdesirable for service providers to be able to use unlicensed RF bands toprovide high speed, high capacity data services. In addition, a serviceprovider may not hold licenses in every geographic area that it desiresto provide service. Therefore, use of the unlicensed frequency bands mayallow a service provider to cover a greater geographical area.

In 1997 the FCC created a wireless arena called Unlicensed NationalInformation Infrastructure (U-NII). System operators are free to operatewireless equipment in three sub-bands (5.15 to 5.25 GHz, 5.25 to 5.35GHz and 5.725 to 5.825 GHz) without acquiring a licensed frequencyspectrum. The FCC specifies the conditions for operating wirelessequipment in the U-NII frequency band. However, operators are notprotected from possible interference from other U-NII operatorstransmitting in the vicinity or even other systems or devices whichutilize the same frequencies.

The IEEE, a standards group, is defining a wireless LAN standard,referred to as IEEE 802.11a for operation in the U-NII band. Equipmentthat conforms to this standard will operate indoors at the lower andmiddle frequency sub-bands (i.e. 5.15 to 5.25 GHz and 5.25 to 5.35 GHz).The ETSI BRAN group in Europe has defined an air interface standard forhigh-speed wireless LAN equipment that may operate in the U-NIIfrequency bands. Equipment that is compatible with this standard maycause interference with use of these unlicensed bands.

A problem that many operators face in the unlicensed bands is a need toprovide the highest possible data rates to subscriber units. One priorart method of providing greater through-put in licensed bands is to havea very large channel bandwidth providing data to the subscriber. Toincrease through-put to a particular subscriber unit, the bandwidth mustnecessarily increase or the modulation scheme that is used has to becomemore complicated. Problematically, bandwidth is limited in theunlicensed bands and using a higher modulation scheme causes an increasein the necessary signal to noise ratio or carrier to interference ratio(C/I). For example, increasing from a quadrature phase shift keying(QPSK) modulation to a 16 quadrature-amplitude modulation (16QAM), whichdoubles the throughput, requires a 6 dB increase in C/I.Problematically, such increases in C/1 may not be practical,particularly in the unlicensed bands where a significant amount ofinterference may be present.

Another problem associated with increasing the capacity or through-putof a RF wireless data transmission system can be the limited amount ortype of hardware available to build a point-to-multipoint wireless datatransmission system. For example, some chip sets are only available witha 6 Mhz channel bandwidth. This limits capacity to what can move througha 6 Mhz channel bandwidth. Likewise, to use a wider bandwidth channel,which would increase capacity, a desired modulation scheme might not befeasible, due to complications and costs in designing a system withoutthe benefit of off-the-shelf chips.

Multiple input multiple output (MIMO) systems divide a single channelinto different transmit streams, but use multiple antennas that are atthe same base station location. Code division multiple access (CDMA)systems use multiple transmit codes from different base stations, usingthe same frequency. The global system for global communications (GSM)system combines multiple timeslots from a single frequency to increasecapacity using a single antenna. Therefore, it is desirable for awireless RF data communication system to use multiple frequencies toincrease capacity.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method for stackingreceiver channels to increase through-put in a wireless RF datatransmission system. To accommodate narrower bandwidths, whileincreasing the through-put to a particular subscriber unit, a pluralityof channels are used to transmit from a hub to the particularsubscriber. The plurality of channels do not necessarily have to beadjacent in the frequency spectrum, easing frequency planning andinterference avoidance scheme constraints. The plurality of channels aretransmitted simultaneously to the subscriber. The channels can either betransmitted through a single antenna using a combining network, orthrough multiple antennas that are pointed in the direction of thesubscriber. The antennas that are used do not necessarily have to havethe same polarization, or be collocated. For example, a subscriber witha wide antenna beam width, or if the subscriber has multiple narrow beamantennas, can communicate with multiple hub sites simultaneously,thereby having a plurality of independent signal paths reaching thesubscriber unit. These two paths can then be combined at the receiver.If the same modulation scheme is used on both channels, the data rate iseffectively doubled for that particular subscriber unit.

However, one of the advantages of a preferred embodiment of the presentinvention is an ability to use a robust modulation scheme on onechannel, and a higher capacity modulation scheme, that requires a higherC/I, on the other channel. The channels are used simultaneously, so themore robust control channel can communicate to the second receiver thetype of modulation to be used for the high capacity channel. In theprior art changing the modulation scheme of a data channel required abursty receiver, which is a more complicated design than a continuousreceiver. For a single channel, a modulation change from, for exampleQPSK to 16QAM, requires that the receiver reacquire the signal. However,in the present invention, the second channel may only utilize a highermodulation scheme such as 16QAM or 64QAM. Alternatively, the secondchannel may use reduced channel coding with the same modulation whichalso increases data throughput. Preferably, when a change in modulationis necessary, the more robust channel, the control channel, prepares thereceiver prior to the change of modulation for the second channel. Thesecond channel does not have to reacquire the signal, because it isprepared for the switch to a higher modulation by the control channel.

As the modulation and/or transmission coding scheme changes depending onchannel conditions, the control channel alerts the subscriber receiverto the modulation and coding system to be used. Additionally, while thesubscriber unit is not receiving data, it can monitor conditions of achannel and alert the hub that it is capable of receiving a very highmodulation or complex modulation scheme on the non-control, datachannel. The hub may then convey a message via the control channel thatthe hub will use such a modulation and prepare the subscriber unit toreceive at the higher modulation. As the channel conditions change dueto fading or due to interference that arises in the network, themodulation scheme that is used on the data channel may change. Therobust control channel may also experience excessive interference. Whenthis occurs the control channel modulation can be lowered to an evenmore robust modulation. Thereby, two degrees of freedom forcommunication to the subscriber are allowed by the preset system, whichprovides a very versatile system. Also, if there is a failure in thesystem resulting in loss of one channel or if interference on one of thechannels becomes unacceptably high, the system can continue tocommunicate via the remaining channel, albeit at a lower data transferrate.

Advantageously, using narrower bandwidth channels rather than higherbandwidth channels to communicate with users, gives a frequency plannermore flexibility in the design of a network, and gives the system moreflexibility in mitigating any interference that might arise. Forexample, if interference occurs on one of the two channels that is beingtransmitted to the user, there are more narrow band channels availablefor the hub to select from for future transmissions. The present systemis well adapted for use in a frequency duplex system, a time duplexsystem, a time division duplexing (TDD) system or other similarduplexing or multiplexing systems.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims. The novel features which are believed to be characteristic ofthe invention, both as to its organization and method of operation,together with further objects and advantages will be better understoodfrom the following description when considered in connection with theaccompanying figures. It is to be expressly understood, however, thateach of the figures is provided for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWING

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawing, in which:

FIG. 1 is a diagrammatic illustration of the present system and methodfor stacking receiver channels in an RF data transmission system; and

FIG. 2 is a flow chart of the present method.

DETAILED DESCRIPTION

FIG. 1 shows system 100, which provides a data communication connectionbetween hub base station 10 and subscriber station 11. In this system,the connection between base 10 and subscriber 11 is through RFtransmission medium 14, preferably employing channels F₁, F₂ and F₃. Theillustrated base station has two transmitters, 103 and 104, and thesubscriber station has two or more receivers 106 and 107. The hub 10 canprovide an enhanced data transmission rate from the hub 10 to thesubscriber 11 employing both hub transmitters 103 and 104 for receptionby subscriber receivers 106 and 107. Transmitters 103, 104 and 105 andreceivers 102, 106 and 107 are designed to transmit and receive using adigital transmission scheme, preferably one whose data rate can bemodified based on channel conditions. For example, BPSK or a higher ratemodulation scheme, including but not limited to QPSK, 16QAM, 64QAM or256QAM may be used. Other modulation schemes, such as multilevel PSK,may be used. Also, OFDM, OFDMA, CDMA or single carrier systems can beused as a transmission system. Illustrated subscriber station 11 hassingle transmitter 105 and base 10 has single receiver 102. However,different architectures could be utilized to provide more receivers andtransmitters at either end to provide symmetric traffic, or conversely,to make the traffic more asymmetric.

Preferably, the basic flow of data comes from network backbone 12, viaconnection 15 to base station 10. Customer premises equipment (CPE) 13receives data via link 16 to subscriber station. Data from internetbackbone 12 may be divided into two streams at box 101 and isre-assembled at box 108 for use by the CPE.

Multiple receivers 106 and 107 allow reception from multiple hubs at anyparticular instant in time. Multiple receivers allow switching to adifferent hub if a first hub is experiencing particularly heavy trafficor if interference conditions dictate such a switch. Also, transmissionsfrom multiple hubs, that might be in different locations, may bereceived simultaneously by such multiple collocated subscriberreceivers. This facilitates flexibility and an ability to decorrelatesignals, which is a benefit for mitigation of deep fades or interferencethat may not occur simultaneously on two different receive paths fromdifferent hubs. Additionally, use of multiple receivers 106 and 107allows switching to an alternative channel or channels for reception ifconditions warrant.

As pointed out above, in a preferred embodiment of the present systemthere are two transmitters 103 and 104 at the base site 10. These twotransmitters preferably use two different channels, F₁ and F₂, whichhave frequencies which may or may not be adjacent in the frequencyspectrum. Additionally, channels F₁ and F₂ may or may not have diversepolarizations. The output of the two transmitters can either be combinedfor output to a single antenna, or two different antennas can be used.The subscriber unit 11 has two different receivers, 106 and 107 withwhich to receive data. The base station may split information from theInternet backbone 12 between the two transmitters, 103 and 104.Alternatively, the data may not be split, depending on the payloadeither of the transmitters can handle. For example, transmitter 103 maybe using a QPSK modulation scheme, which provides half the data ratethat transmitter 104 would provide if utilizing 16QAM.

Accordingly, preferred more robust system 100 utilizes a lowermodulation scheme on one channel, such as channel F₁, to insurecommunication and to direct control of modulation and/or transmissioncoding of the other channel, F₂. For example, receiver 106 may generallyreceive using a lower modulation scheme; it will alert receiver 107 whattype of modulation and/or transmission coding will be used bytransmitter 104. So if the channel conditions allow, a very highmodulation scheme such as 256QAM, may be transmitted from transmitter104 to receiver 107. Then receiver 106 can alert receiver 107 to changemodulations. If interference arises in the network, the modulationscheme used by transmitter 104 may be decreased, for example down tousing QPSK. Since transmitter 103 is also still using QPSK, in ourexample though-put of the system will still double the through-put of asingle transmitter/receiver network QPSK. Since, when a change inmodulation and/or transmission coding is to occur, receiver 106 alertsreceiver 107 that change is going to occur, second receiver 107 iscontinually prepared for modulation and or coding changes.Advantageously, receivers 106 and 107 may be continuous receivers.Therefore, the design of receivers 106 and 107 do not have to be ascomplex as the design for a bursty receiver which would be required tomake changes without the benefit of alerts.

Signal quality or metrics may be monitored at subscriber station 11.Signal metric analysis may be carried out by subscriber station 11 atbox 109. A metric report transmitted from Subscriber station 11 to basestation 10 may be employed by modulation control 110 to make decisionsas to the modulations to be used by both control channel F₁ and datachannel F₂. The Modulation control instructs transmitters 103 and 104 asto which modulations to use and which modulation to prepare receiver 107to receive.

Timing can be continuously maintained on more robust channel F₁. Thisfrees higher capacity channel F₂ of the need to maintain timing. Therelay timing can be maintained on second receiver 107 through the firstreceiver 106. In addition to maintaining the timing, control andsynchronization information necessary for the subscriber unit to operatecan be transmitted on control channel F₁. Therefore, transmitter 104 mayneed only transmit data bits and may not need to also transmitsynchronization and control protocol bits. Thereby, reducing overheadand increasing data through-put on the data channel F₂.

With attention directed to FIG. 2, preferred method 200 of practicingthe present invention is charted. At box 201 a hub preferably utilizes alower modulation scheme on one channel, such as a control channel, toinsure continuous communication and to direct control of modulationand/or coding of at least one data channel. A subscriber stationpreferably receives the control channel using the lower more robustmodulation scheme and directs at least one other receiver as to whatmodulation and/or coding to employ to receive the transmitted data inresponse to control information transmitted on the control channel, box202. If channel conditions allow, as monitored by the subscriber stationat box 203, the subscriber station may request an increase in themodulation and/or change in the transmission coding scheme at box 204.In response the hub may direct an increase in modulation and/or codingvia the control channel, at box 205. Timing for the data channel may bemaintained on the more robust control channel, box 206. Additionally, oralternatively, at a box 206, control and synchronization bits requiredto decode the data may be included in the information transmitted on thecontrol channel.

The bandwidth of channels F₁ and F₂ do not necessarily have to be thesame. The present system provides flexibility by permitting the use ofmultiple frequencies to avoid or mitigate interference problems. Also,the present system allows the use of off-the-shelf chip sets. Forexample, it might be desirable to use a 20 Mhz channel bandwidth to havehigh data rate, but if only 10 Mhz wide channel bandwidth chip sets areavailable, it is advantageous to stack two of the narrower band chipsets and employ the present system and method in order to increasecapacity of a system.

Generally, Internet traffic is asymmetric, that is to say, there is moretraffic coming from hub site 10 to subscriber 11, than from subscriber11 to hub 10. Therefore, in present system 100 only transmitter 105 isshown transmitting from the subscriber to receiver 102 at the hub site10, via channel F₃. Preferably, a robust modulation is used for channelF₃ transmissions from the subscriber 11 to the hub 10 because a higherdata rate is not generally needed in the subscriber 11 to hub 10direction. However, present system 100 is adaptable to adopt a highcapacity data transmit channel scheme in the reverse direction just asdisclosed in the forward direction, if desired.

Additionally, the present system provides a degree of system back-up. Ifthere is a hardware failure at subscriber 11, for example if receiver107 were to become inoperable, transmissions could still take place at alower data rate through receiver 106.

While two transmitters 103 and 104 are shown at base station 10, anynumber of channels and transmitters can be used. Preferably, all thedata transmission channels would key off of one control channel havingone modulation scheme. The other channels could have higher modulationsor different modulation schemes, as desired, all controlled from onetiming and control signal sent on the first channel.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

What is claimed is:
 1. A wireless RF data transmission systemcomprising: a first station transmitting data on a plurality ofchannels, said plurality of channels comprising a data channel and acontrol channel; a second station receiving said data transmitted fromsaid first station on said data channel and said control channel, saidsecond station adjusting demodulation of said data channel in responseto instructions included in said control channel; wherein said controlchannel uses a modulation scheme with a lower requiredcarrier-to-interference ratio (C/I) than a modulation scheme used bysaid data channel; and wherein said data channel and control channel aretransmitted on non adjacent frequency bands, each of said frequencybands comprising a plurality of channels; wherein said control channelcarries synchronization information for said data channel; and whereinsaid control channel provides a data path when said data channel isunavailable.
 2. A method for wireless RF data transmission comprising:transmitting data on a data channel from a first station using a datamodulation scheme; receiving feedback at said first station of channelconditions for said data channel at a second station; changing saidmodulation scheme used by said data channel in response to a change insaid channel conditions; and communicating said modulation change usinga control channel, wherein said communicating occurs before saidchanging; wherein said control channel uses a modulation scheme with alower required carrier-to-interference ratio (C/I) than a modulationscheme used by said data channel; and wherein said data channel andcontrol channel are transmitted on non adjacent frequency bands, each ofsaid frequency bands comprising a plurality of channels; andtransmitting data on said control channel when said data channel isunavailable.